Wells are generally drilled into the ground to recover natural deposits of hydrocarbons and other desirable materials trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a “drill string.” The drill string is a long string of sections of drill pipe that are connected together end-to-end. Drilling fluid, or mud, is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the borehole wall.
In conventional drilling, a well is drilled to a selected depth, and then the wellbore is typically lined with a larger-diameter pipe, usually called casing. Casing typically consists of casing sections connected end-to-end, similar to the way drill pipe is connected. To accomplish this, the drill string and the drill bit are removed from the borehole in a process called “tripping.” Once the drill string and bit are removed, the casing is lowered into the well and cemented in place. The casing protects the well from collapse and isolates the subterranean formations from each other.
Conventional drilling typically includes a series of drilling, tripping, casing and cementing, and then drilling again to deepen the borehole. This process is very time consuming and costly. Additionally, other problems are often encountered when tripping the drill string. For example, the drill string may get caught up or stuck in the borehole while it is being removed. These problems require additional time and expense to correct.
FIG. 1A shows a prior art drilling operation. A drilling rig 2 and rotary table 4 at the surface are used to rotate a drill string 6 with a drill bit 8 disposed at the lower end of the drill string 6. The drill bit 8 drills a borehole 10 through subterranean formations that may contain oil and gas deposits. Typically, an MWD (measurement while drilling) or LWD (logging while drilling) collar 12 is positioned just above the drill bit 8 to take measurements relating to the properties of the formation as the borehole 10 is being drilled. In this description, MWD is used to refer either an MWD system or an LWD system. Those having ordinary skill in the art will realize that there are differences between these two types of systems, but the differences are not germane to the embodiments of the invention.
The term “casing drilling” refers to using a casing string as a drill string when drilling. A bottom hole assembly (“BHA”), including a drill bit, is connected to the lower end of a casing string, and the well is drilled using the casing string to transmit drilling fluid, as well as axial and rotational forces, to the drill bit. Casing drilling enables the well to be simultaneously drilled and cased.
FIG. 1B shows a prior art casing drilling operation. A rotary table 14 at the surface is used to rotate a casing string 16 that is being used as a drill string. The casing 16 extends downwardly into borehole 18. A drill bit 20 is connected to the lower end of the casing string 16. When drilling with casing, the drill bit 20 must be able to pass though the casing string 16 so that the drill bit 20 may be retrieved when drilling has been completed or when replacement or maintenance of the drill bit 20 is required. Thus, the drill bit 20 is sized smaller than the inner diameter of the casing string 16.
The drill bit 20 drills a pilot hole 22 that must be enlarged so that the casing string 16 will be able to pass through the borehole 18. An underreamer 24 is positioned below the casing string 16 and above the drill bit 20 so as to enlarge the pilot hole 22. A typical underreamer 24 can be positioned in an extended and a retracted position. In the extended position, the underreamer 24 enlarges the pilot hole 22 to the underreamed borehole 18, and in the retracted position (not shown), the underreamer 24 collapses so that it is able to pass through the inside of the casing string 16.
FIG. 1B also shows an MWD collar 26 positioned above the drill bit 20 and the underreamer 24, but below the casing string 16. The MWD collar 26 takes measurements related to formation properties as drilling is taking place. It should be noted that other positions of these BHA components are possible and are not limited to the figures shown.
Casing drilling eliminates the need to trip the drill string before the well is cased. The drill bit may simply be retrieved by pulling it up through the casing. The casing may then be cemented in place, and then drilling may continue. This reduces the time required to retrieve the BHA and eliminates the need to subsequently run casing into the well.